Processes for the production of polymethylene polyphenyl polyamines and phosgenation of such polyamines to produce the corresponding polyisocyanates are well known in the art (see, e.g., U.S. Pat. Nos. 3,253,031, 3,260,751, 3,277,139, 3,277,173, 3,362,979, 3,496,229, 3,517,062, 3,641,094, 3,912,600 and 4,259,526).
In general, the process of producing the polyisocyanates includes the steps of phosgenating the polyamines in solution in an organic solvent, removing excess phosgene and then stripping the inert solvent. Some effort has been expended in reducing the color of the resultant polyisocyanates. U.S. Pat. No. 4,465,639 describes the addition of water prior to the solvent stripping step to reduce the color. U.S. Pat. No. 4,792,624 describes a specific polyamine recycling step during the aniline/formaldehyde reaction to cause a reduction in color of the corresponding polyisocyanate.
Czechoslovakian Patent 154,431 describes a method of isolating isomers of diaminodiphenylmethane using a water extraction process. The reference describes the use of deoxygenated water in the extraction and suggests that reducing agents should be added to the water in order to minimize staining of the isolated diamine.
U.S. Pat. No. 5,312,971 describes a process whereby a reducing agent such as borane-tetrahydrofuran added to the polyamines can result in reduction of color of the corresponding isocyanate.